A coated type magnetic recording medium prepared by providing a non-magnetic support with a magnetic recording layer containing magnetic particles dispersed in a binder, followed by drying the magnetic recording layer has been widely used. Recently, attention has been directed to a thin metal film type magnetic recording medium comprising, as a magnetic recording layer, a thin ferromagnetic metal film prepared by a method wherein the film is formed in a vacuum chamber, e.g., by vacuum deposition, sputtering or ion-plating, or by a metal plating method such as electric-plating or electroless plating and has become of high practical value with increases in the demand for high density recording.
A thin metal film type magnetic recording medium is very effective in view of electromagnetic properties and is suitable for high density recording because ferromagnetic metal having high saturation magnetization can be fabricated into a very thin film without using a non-magnetic substance such as a binder.
However, one serious problem of the thin metal film type magnetic recording medium is that, with the passage of time after preparing the medium, the surface of metal film easily corrodes to lose its original excellent electromagnetic properties. To solve this problem, it has been proposed to provide a protective layer on a metal surface by coating a thermoplastic polymer as described in, for example, German Patent Application (OLS) No. 2,220,964 and No. GB-2106011A or a thermosetting polymer as described in, for Example, German Patent Application (OLS) No. 3,220,066 and Japanese Patent Application (OPI) No. 158037/82 (the term "OPI" as used herein means a "published unexamined Japanese patent application") on the metal surface. However, according to the method, the thickness of the protective layer is limited in that a spacing loss occurs between a magnetic head and a magnetic layer as the thickness increases, and hence satisfactory anti-corrosive properties cannot be achieved. It has been also known that anti-corrosive effect can be improved by nitrating the surface of the magnetic layer as disclosed in Japanese Patent Application (OPI) No. 33806/75 or by oxidizing the surface thereof as disclosed in Japanese Patent Publication No. 20025/67 corresponding to Canadian Pat. No. 846,765. However, these methods generally require a relatively long treating time, for example, from 10 minutes to 2 hours, and therefore with a short treating time, a satisfactory anti-corrosive effect cannot easily be obtained.
Another serious problem of the thin metal film type magnetic recording medium is that running properties and durability are poor, that is, a magnetic tape tends to stick to a tape path of magnetic tapes when the tape is made running on a tape deck and running is stopped or that the durability during still mode use is poor. To solve the above problem, a method has been proposed to provide a lubricating agent, such as a thermoplastic polymer, a thermosetting polymer, a fatty acid as described in, for example, Japanese Patent Application (OPI) No. 68930/81, or a fatty acid ester as described in, for example, Japanese Patent Application (OPI) No. 80827/81 on the thin ferromagnetic metal film. However, in this method, running properties or durability are still not completely satisfactory. To achieve satisfactory running properties and durability, the layer containing a lubricating agent must be made thick, thereby losing the electromagnetic properties due to spacing loss. That is, the thickness of the layer containing a lubricating agent must be thick in order to obtain satisfactory anti-corrosive effect, which is not preferable for electromagnetic properties.
The inventors of the present invention have made extensive studies and research, and as result, have finally reached the present invention.